In the web application the reference handling is a bit special because we have to distinguish the case of an already generated reference - which is a text value that can be modified by the user if the field REFFIXED is 0 - and the case of a reference to be generated - which is an integer value corresponding to the list of references.
When you pick a reference in the drop-down list of references, the web page sends a CEB_REFERENCE value to the back-end. This value gets copied to the REFERENCE field, and the REFFIXED value is put to 2 to indicate that the reference field contains the ID of the reference to be generated.
You can mimic this behavior during the creation of the entity. Just add
_reference=14&_reffixed=2 to your initialisation URL.